Chemical-mechanical polishing (CMP) is a widely used means of planarizing silicon dioxide as well as other types of layers on semiconductor wafers. Chemical mechanical polishing typically utilizes an abrasive slurry disbursed in an alkaline or acidic solution to planarize the surface of the wafer through a combination of mechanical and chemical action. A typical chemical mechanical polishing tool includes a rotatable circular platen or table on which a polishing pad is mounted and a polishing device is positioned above the pad. The polishing device includes one or more rotating carrier heads to which wafers can be secured typically through the use of vacuum pressure. In use, the platen is rotated and an abrasive slurry is disbursed onto the polishing pad. Once the slurry has been applied to the polishing pad, a downforce is applied to each rotating carrier head to press its wafer against the polishing pad. As the wafer is pressed against the polishing pad, the surface of the wafer is mechanically and chemically polished.
As semiconductor devices are scaled down, the importance of chemical-mechanical planarization to the fabrication process increases. In particular, it becomes increasingly important to control removal rate variations between wafers. Variations in removal rates effect the thicknesses of layers being polished. Variations in layer thicknesses deleteriously impact subsequent fabrication steps, such as lithography, and degrade device performance. To further complicate matters, the complexity of polishing tools is also increasing. For example, chemical mechanical polish tools have moved from single-arm polishing tools to multi-arm polishing tools which can, for example, polish five wafers simultaneously. The use of multiple polishing arms further contributes to variations in removal rates between wafers. As a result of the increasing importance and complexity of chemical mechanical planarization, semiconductor manufacturers seek systems and methods for controlling chemical mechanical polish tools.